To measure the resistance of a load, an ohmmeter supplies a current having a known magnitude through a pair of output leads to the load. The voltage drop across the load as a result of ohmmeter current flowing therethrough is measured to drive an output which is calibrated to display resistance in ohms. However, because ohmmeter current flows both though the leads interconnecting the ohmmeter terminals and through the load, lead resistance, which is on the order of 0.05-0.1 ohm, contributes to error in the reading.
Precision laboratory ohmmeters avoid this source of error using a four lead output configuration that includes a pair of leads, termed "HI OUT" and "LO OUT" leads for supplying ohmmeter current to the load and another pair connected directly across the load to sense voltage. The second pair of leads, termed "Sense" leads, enable the voltage monitored by the ohmmeter terminals to ignore the voltage drop associated with ohmmeter leads themselves. Other types of ohmmeters, however, have two lead output configurations that include no lead resistance error elimination circuitry.
Laboratory ohmmeters and other instrumentation are calibrated following manufacture and from time to time thereafter by connecting the output leads of the ohmmeter to a standard or reference resistor having a very precisely known value of resistance, and adjusting the reading of the ohmmeter display to match the resistance value of the reference. Four lead ohmmeters are calibrated easily because the sense leads of the ohmmeter compensate for lead resistance between the ohmmeter terminals and the reference resistor. Calibration of two-lead ohmmeters, however, is more troublesome, because no convenient means to compensate or eliminate the effect of lead resistance on calibration of two-lead ohmmeters is known.
Accordingly, one object of the invention is to provide lead resistance compensation in instrumentation calibration equipment.
Another object of the invention is to provide new and improved ohmeter compensation, wherein the effect of lead resistance on ohms calibration is eliminated.
Another object is to provide a method of and circuitry for calibrating two-lead ohmmeters, wherein effect of lead resistance on ohms calibration is eliminated.
A further object of the invention is to provide new and improved electronic circuitry for lead resistance compensation of a two-lead ohms calibrator.